When a frequency converter or similar device is used for controlling the motion of a motor, it is desirable to know the motor parameters. The machine parameters can be used in control algorithms of the converter for accurate control of the load. The parameters of the machine can be identified using an identification procedure during the commissioning of the drive. In this identification run, the converter can perform one or more tests on the machine and can estimate the parameters to be used for control.
In some situations, the load connected to the machine can produce some restrictions to the drive so that identification procedure cannot be carried out. The load connected to the controlled motor can produce counter torque which disturbs the identification run to the extent that identification procedure does not provide accurate results enabling precise control of the motor. It might also happen that the identification run cannot be performed with the load at all due to maximum speed and/or maximum torque limits set for the load. In these situations, the identification run can involve the controlled machine being completely taken off from the load. This can be cumbersome and sometimes even impossible if the machine to be controlled is an integral part of the mechanics of the load.
For these purposes, frequency converters may have an option for a stand-still identification procedure in which the rotor of the machine is not rotated. However, the stand-still procedures identify only some of the parameters of the motor while other parameters are calculated using rated values of the machine.
FIG. 1 shows an L-equivalent circuit of an induction machine. The stator resistance Rs of the equivalent circuit can be quite easily identified using DC-magnetization current (i.e., stator current is) produced with a constant stator voltage us. Rotor resistance RR and stray inductance σLs can be identified using a DC-current with summed pulsed current or with some other injected current.
In known stand-still identification methods, the measurement of main inductance LM and the rotor time constant τr (=LM/RR) depending thereon can be very tricky because the machine cannot be rotated. This is due to the fact that in stand-still methods, the stator current pulses used are almost entirely summed with opposing phases to the rotor current. As such, corresponding changes in the magnetizing current im are relatively small and thus the response from the main inductance to the test pulses is poor.
The known stand-still methods cannot reach satisfying estimates for the main inductance and rotor time constant and these parameters have been approximated from a slip frequency and a power factor calculated from the rated values of the machine or from a cos θ value given as rated value. However, the rated values are not necessarily exact so that the values obtained for main inductance and rotor time constant with the known stand-still identification methods are inaccurate which is reflected as poorer performance of the control when compared with that with parameters obtained with identification runs performed with rotating machines.
The accuracy of the voltage measurement should be increased considerably so that sufficiently accurate estimates for the main inductance LM or rotor time constant τr could be calculated in stand-still methods. The increase in voltage accuracy is hard to achieve because, due to reduced costs, the output voltage in frequency converters has been calculated using a measured DC bus voltage and output switch combination. In this kind of measurement, the commutation delays and threshold voltages cause inaccuracies to the voltage measurement. These inaccuracies can be quite considerable when compared with the voltage response from the main inductance in the injection frequency.
The rotor flux can be estimated during DC magnetization using a voltage model represented as:
                                          ψ            R                    ⁡                      (            t            )                          =                                            ∫                              t                0                            t                        ⁢                                          (                                                      u                    sd                                    -                                                            R                      s                                        ⁢                                          i                      sd                                                                      )                            ⁢                              ⅆ                t                                              -                      σ            ⁢                                                  ⁢                          L              s                        ⁢                          i              sd                                                          (        1        )            when the magnetization of the machine, which is initially free from magnetization, is started at time instant t0. During the magnetization, the magnetizing current increases as the rotor current decreases when the stator current is kept constant. As seen for example from FIG. 1, the stator current equals the difference between the magnetizing current and the rotor current.
The rotor flux increases with the increasing magnetizing current according to:ψR(t)=LMim(t)  (2)
If the DC magnetization is stopped at time instant t1, the main inductance can be calculated as a quotient between the rotor flux estimate ψR (t1) calculated with equation (1) and the magnetizing current:
                              L          M                =                                            ψ              R                        ⁡                          (                              t                1                            )                                                          i              m                        ⁡                          (                              t                1                            )                                                          (        3        )            
The value of the magnetizing current im(t1) at time instant t1 in equation (3) is uncertain. In known methods, the magnetizing current is attempted to be calculated from the current and/or voltage responses during the magnetization, when other parameters of the machine are known or when the other parameters are estimated from the mentioned responses together with the main inductance LM. Due to eddy currents formed in the machine during transients and skin effect in the rotor windings, which the simple L-equivalent circuit does not take into account, the above methods are inaccurate. Further inaccuracies can be caused by saturation of inductances, which should be taken into account when estimating the value of magnetizing current from the current and/or voltage waveforms during the DC magnetization.
Modeling of the machine in the transient state could alternately use a magnetizing time that is so long that the machine is in a steady state before stopping the magnetization at time instant t1. In the steady state, the value of magnetizing current corresponds to that of the stator current so that the main inductance LM could be estimated as a quotient between the rotor flux estimate and the DC current reference used in magnetization; that is:
                              L          M                ≈                                            ψ              R                        ⁡                          (                              t                1                            )                                            i                          d              ⁢                                                          ⁢              c              ⁢                                                          ⁢              _              ⁢                                                          ⁢              ref                                                          (        4        )            
However, in equation (4); the magnetization time t1−t0 should be at least four times the rotor time constant so that the DC current reference would correspond closely enough to the magnetization current at time instant t1. When such a long magnetizing time is used, cumulative errors of the voltage model can decrease the accuracy of the rotor flux estimate ψR(t1) especially in connection with larger machines, which have a large rotor time constant. With smaller machines, the steady state can be achieved faster, but because of their larger stator resistance, the inaccuracy of the voltage model is higher. In summary, methods based on voltage model (1) and steady state equation (4) do not produce accurate estimates on machine parameters.